Moderator

Click 'User CP' on the Smogon toolbar; it's the most left option. Then click 'Edit Avatar' under 'Settings & Options'. You can then upload an avatar of your choice.

Note: The maximum size of your custom image is 150 by 150 pixels or 30.0 KB (whichever is smaller).

In the future, you should look at 'FAQ' instead of posting a question about this. FAQ is on the Smogon toolbar, next to 'User CP.' FAQ has all the information about issues like modifying your Smogon profile.